1. Field of the Invention
The invention concerns an improvement to the method of stamping in a liquid female tool, and an apparatus for carrying out the method.
2. Description of the Prior Art
The conventional stamping process comprises mechanically deforming a flat element, described as a blank, by means of a punch and a female tool, to obtain an article in the form of a case or socket. The punch pushes back the central part of the blank at the bottom of the female tool and entrains the edge of the blank, or flange, which is "swallowed up" in the space between the punch and the female tool. The flange, which is generally circular with a maximum diameter F, thus forms the cylindrical lateral wall of the article of diameter D, corresponding to that of the punch. In order to prevent the flat flange from forming any folds during its deformation, it is necessary to guide it closely. In particular, the flange must initially be applied to the upper flat surface of the female tool by a blank holder.
It has been known for a long time that the method has been improved and has given rise to the process of stamping in a liquid female mold. The improved process is described e.g. on pages 17-23 of the revue "Machine Moderne" of September 1966, where it is described as "hydro-mechanical stamping".
In the process of stamping in a liquid female tool (or hydro-mechanical stamping), the metal female tool is replaced by a chamber full of liquid, in which the blank can be lowered when it is pushed by the punch. The liquid is put under pressure and applies the blank to the surface of the punch during its displacement. The liquid under pressure thus takes on the role of the female tool. As in conventional stamping, the flange of the blank must however be guided mechanically during its deformation. For this purpose, the free, upper section of the chamber is closed by a stamping ring which contains an aperture of a section corresponding to that of the stamped article, i.e. the largest section of the punch plus the thickness of the blank, with the clearance necessary to give passage to the metal of the flange when it is entrained to form the lateral wall of the article.
In the known process of stamping in a liquid female tool, the liquid level must be exactly flush with the upper level of the ring. When the blank is placed on the stamping ring at the beginning of the stamping operation, it is thus very exactly aligned with the level of the liquid, without leaving an air pocket of any size. As in the conventional method, the flange of the blank is pressed onto the ring by a blank holder.
As soon as the central part of the blank is depressed by the punch within the chamber, through the stamping ring, the liquid is put under pressure and tends to escape, flowing back between the flange of the blank and the stamping ring. The flange is lifted against the blank holder, allowing the liquid to escape between its lower surface and the stamping ring.
The pressure in the chamber and the rate at which the liquid escapes are determined at any moment by the speed at which the punch descends, the pressure applied by the blank holder and the width of the portion of flange still applied to the blank holder, which forms a seal.
The fact that the central part of the blank is constantly applied to the punch by the pressure of the liquid reduces the danger of the cylindrical, lateral wall folding during its formation. Cylindrical stamping ratios F/D of the order of 2.7 are permissible; these could not possibly be obtained by conventional stamping.
The process also enables conical or even ogival articles, which would require several passes by the conventional method, to be obtained in a single operation. And finally, the machinery is relatively simplified, since all that is required is to machine a thin stamping ring, rather than a female tool equal in height to the article to be stamped.
In spite of all these advantages, the process of stamping in a liquid female tool is still little used. In fact it requires care and special precautions. The free part of the blank inside the stamping ring has to be applied from the beginning of deformation and at all points to the liquid surface without leaving any air pocket. The liquid level must be precisely flush with the upper plane of the stamping ring, and this must be so at the beginning of every operation.
The precisely must entrain the flange within the chamber very regularly and symmetrically to form the lateral wall of the article stamped. The flange must thus be gripped and decelerated below the blank holder very regularly during its deformation. It is important that the surfaces of both the stamping ring and the blank holder should be cleaned carefully, to eliminate any drop of liquid which would result in unevenness and often even in a fracture of the blank.
Finally, as in conventional stamping, the blank must be lubricated very homogeneously to facilitate its sliding against the stamping ring and blank holder, from the very beginning of the stamping process, before the liquid can escape between the ring and the flange and act as a lubricant.
Unless a fairly complex apparatus is used to control the pressure by a pump and control valve, the pressures in the chamber will generally be very high, of the order of 1000 bars, which will increase the power requirement of the press.
It is also possible to fit a circular seal integral with the stamping ring and providing imperviousness between the flange and the stamping ring, then to control the pressure in the chamber solely by a pump and valve. However, the radial sliding of the flange on the upper surface of the seal is tricky and there are many hitches.